1. Field of the Invention
The present invention relates in general to an apparatus and method for removing nitrogen oxides and particulate from a flue gas, and in particular to a catalytic electrostatic precipitator which reduces nitrogen oxides with ammonia and simultaneously removes particulate matter from the flue gas.
2. Description of the Related Art
Selective catalytic reduction (SCR) systems catalytically reduce flue gas nitrogen oxides (NO.sub.x) to nitrogen (N.sub.2) and water (H.sub.2 O) using ammonia (NH.sub.3) in a chemical reduction. The term ammonia as used herein is meant to include aqueous ammonia or anhydrous ammonia as well as an ammonia reagent or precursor, like urea, or mixture thereof. This technology is an effective method of reducing NO.sub.x missions especially where high removal efficiencies (70%-90%) are required. Environmental considerations will likely require this technology on many installations during the upcoming year.
The NO.sub.x reduction reactions take place as the flue gas passes through a catalyst chamber in an SCR reactor. Before entering the catalyst, ammonia is injected into and mixed with the flue gas. Once the mixture enters the catalyst, the NO.sub.x reacts with ammonia as represented by the following equations: EQU 4NO+4NH.sub.3 +O.sub.2 .fwdarw.4N.sub.2 +6H.sub.2 O (I) EQU 2NO.sub.3+ 4NH.sub.3 +O.sub.2 .fwdarw.2N.sub.2 +6H.sub.2 O (II)
The SCR reactions take place within an optimal temperature range. A variety of catalysts are available and known in this art. Most can operate within a range of 450.degree. F. to 840.degree. F. (232.degree. C. to 499.degree. C.), but optimum performance occurs between 675.degree. F. to 840.degree. F. (357.degree. to 499.degree. C.). The minimum temperature varies and is based on fuel, flue gas specifications, and catalyst formulation. In addition, this minimum temperature tends to increase the flue gas sulfur dioxide content. This results in a smaller operating range as sulfur content increases in order to eliminate the formation of ammonium sulfate salts in a catalyst bed. Above the recommended temperature range, a number of catalyst materials tend to become less effective.
Catalyst material typically falls into one of three categories: base metal, zeolite, and precious metal.
Most of the operating experience to date has been with base metal catalysts. These catalysts use titanium oxide with small amounts of vanadium, molybdenum, tungsten or a combination of several other chemical agents. The base metal catalysts are selective and operate in the specified temperature range. The major drawback of the base metal catalyst is its potential to oxidize SO.sub.2 to SO.sub.3 ; the degree of oxidation varies based on catalyst chemical formulation. The quantities of SO.sub.3 which are formed can react with the ammonia carryover to form the ammonium sulfate salts as previously discussed. They also can react with SO.sub.2 so sulfites and bisulfites are formed.
Most modern SCR systems use a block type catalyst which is manufactured in the parallel plate or honey-comb configurations. For ease of handling and installation, these blocks are fabricated into large modules.
Each catalyst configuration has its advantages. The plate type unit offers less pressure drop and is less susceptible to plugging and erosion when particulate-laden flue gas is treated in the SCR reactor. The honey-comb configuration often requires less reactor volume for a given overall surface area. The catalyst is housed in a separate reactor which is located within the system. At a set location, the catalyst permits exposure to proper SCR reaction temperatures.
In general, the stoichiometry of NO.sub.x reduction is a 1:1 mole ratio of NH.sub.3 to NOx. Based on the stoichiometry, for example, a theoretical mole ratio of 0.80 is required for 80% NO.sub.x removal. However, the actual mole ratio required is slightly higher to account for unreacted ammonia carryover from the reactor (NH.sub.3 slip). Some systems employ a continuous emission monitoring system (CEM) to monitor all atmospheric pollutants. Data generated from the CEM system can be used to control the ammonia flow while achieving the required NO.sub.x emissions level.
The design of each SCR system is unique. The major items to be considered include space constraints, location of existing equipment, temperature requirements, fuel and cost. One location for the SCR reactor is downstream from a boiler or combustion source and upstream of an air preheater which is upstream of a particulate collection device. Another possible location for the SCR reactor is downstream of the particulate collection device immediately after some form of heat exchanger. It is also known to employ an SCR reactor in a combined cycle heat recovery steam generator location (HRSG) .
U.S. Pat. No. 4,871,522 discloses the use of a combined catalytic baghouse and heat pipe air heater. This patent describes catalytically coating surfaces of the heat pipe air heater which is located downstream of the catalytic baghouse for NO.sub.x removal.
Electrostatic precipitators (ESP) are devices known in the art that electrically charge the ash particles in a flue gas to collect and remove them. The unit includes a series of parallel vertical plates through which the flue gas passes. Centered between the plates are charging electrodes which provide an electric field. FIG. 1 is a plan view of a typical ESP section which indicates the above-process arrangement. U.S. Pat. No. 4,888,158 describes modifications made to an electrostatic precipitator which allow for an alkaline slurry to be sprayed therein for the removal of sulfur oxides (SO.sub.x) with the use of a droplet impingement device.
There still exists a need for an integrated electrostatic precipitator which allows for the injection of ammonia with a catalytic reduction of the NO.sub.x while particulates are simultaneously removed at the collector plates. Preferably, the collector plates would be catalytically coated with an SCR catalyst or alternately constructed of the SCR catalyst.